JP7159594B2 - Manufacturing method of grain-oriented electrical steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet.

A grain-oriented electrical steel sheet is a steel sheet containing about 0.5 to 7% by mass of Si and having crystal orientations of {110}<001> orientation (Goss orientation). Grain-oriented electrical steel sheets are used as core materials for transformers and other electrical equipment as soft magnetic materials. A catastrophic grain growth phenomenon called secondary recrystallization is used to control the crystal orientation of grain-oriented electrical steel sheets.

A method for producing a grain-oriented electrical steel sheet is as follows. The slab is heated and hot rolled to produce a hot rolled steel sheet. The hot-rolled steel sheet is annealed as necessary. The hot-rolled steel sheet is pickled as necessary. The pickled hot-rolled steel sheet is cold-rolled at a cold-rolling rate of 80% or more to produce a cold-rolled steel sheet. The cold-rolled steel sheet is decarburized and annealed to develop primary recrystallization. After the decarburization annealing, the cold-rolled steel sheet is subjected to finish annealing to develop secondary recrystallization. A grain-oriented electrical steel sheet is manufactured by the above steps.

A grain-oriented electrical steel sheet is required to have magnetic properties, particularly excellent excitation properties and core loss properties. B8, which is the magnetic flux density at a magnetic field strength of 800 A/m, is used as an index indicating the excitation characteristics of a grain-oriented electrical steel sheet. W _17/50 , which is the iron loss per unit mass when magnetized to 1.7 T at 50 Hz, is used as an index indicating the iron loss characteristics of a grain-oriented electrical steel sheet.

In recent years, there has been an increasing demand for further improvements in iron loss properties of grain-oriented electrical steel sheets. This is because the efficiency of generators and transformers is increased by further reducing core loss of grain-oriented electrical steel sheets.

Iron loss consists of hysteresis loss and eddy current loss. Hysteresis loss is affected by the purity, internal strain, crystal orientation, and the like of the grain-oriented electrical steel sheet. The eddy current loss is affected by the electrical resistance, thickness, grain size, size of the magnetic domain, tension of the film formed on the surface of the steel sheet, and the like of the grain-oriented electrical steel sheet.

If the Si content is increased, the electrical resistance of the steel sheet increases, thereby reducing the eddy current loss. Therefore, it is considered effective to increase the Si content in order to reduce iron loss. However, when the Si content is increased, there are cases where the desired core loss characteristics corresponding to the Si content cannot be obtained.

Techniques for improving iron loss characteristics in grain-oriented electrical steel sheets with an increased Si content have been proposed in Japanese Patent Application Laid-Open Nos. 2001-192733 (Patent Document 1) and 5-345921 (Patent Document 2). there is

In the method for producing a grain-oriented electrical steel sheet disclosed in Patent Document 1, in mass %, Si: 3.0 to 3.8%, Mn: 0.03 to 0.45%, S, Se: single or combined After heating an electrical steel slab containing 0.15% or less, acid-soluble Al: 0.015 to 0.035%, and N: 0.0035 to 0.012% to a temperature of 1250 ° C. or less After rolling, hot-rolled sheet annealing, cold rolling to obtain the final sheet thickness, decarburization annealing, nitriding treatment, and finish annealing are then performed. Then, when the thickness of the hot-rolled sheet is tA (mm) and the thickness of the final cold-rolled sheet is tC (mm), tA/tC is 3 according to the Si content (Si (%)). .57−0.43×Si(%)≦ln(tA/tC)≦4.58−0.64×Si(%). In Patent Document 1, by adjusting the cold rolling rate (tA/tC) within the range of the above formula, the Goss orientation in the primary recrystallization is effectively increased, thereby increasing the Goss orientation density in the secondary recrystallization. It is described that it is possible to increase the Si content, and as a result, it is possible to obtain iron loss characteristics according to the Si content.

In the method for producing a grain-oriented electrical steel sheet disclosed in Patent Document 2, in mass%, C: 0.090% or less, Si: 2.5 to 4.5%, Mn: 0.03 to 0.15% , S: 0.010 to 0.050%, acid-soluble Al: 0.010 to 0.050%, N: 0.0045 to 0.012%, Sn: 0.03 to 0.5%, Cu: 0 An electrical steel slab containing 0.02 to 0.3% Ni, 0.05 to 1.0% Ni, and the balance being Fe and unavoidable impurities is heated to 1250 ° C. or higher, hot rolled, precipitation annealed, and finally Cold rolling with a cold rolling rate of 80% or more, decarburization annealing, and finish annealing are applied. Patent Document 2 describes that magnetic properties (magnetic flux density and iron loss) are enhanced by further containing Ni in the chemical composition of grain-oriented electrical steel sheets with a high Si content.

Japanese Patent Application Laid-Open No. 2001-192733 JP-A-5-345921

However, when a grain-oriented electrical steel sheet having a Si content of 3.30% or more is produced by the methods of Patent Documents 1 and 2, sufficient iron loss characteristics may not be obtained. In particular, when a grain-oriented electrical steel sheet having a Si content of 3.30% or more is manufactured by the manufacturing methods proposed in these patent documents, the iron loss may decrease due to magnetic aging.

An object of the present disclosure is to provide a method for producing a grain-oriented electrical steel sheet that can suppress the occurrence of magnetic aging and obtain excellent iron loss properties even if the Si content is 3.30% or more. .

A method for manufacturing a grain-oriented electrical steel sheet according to the present disclosure includes a hot rolling process, a cold rolling process, an annealing process before final cold rolling, a decarburization annealing process, an annealing separator application process, and a finish annealing process. Prepare.
In the hot rolling process, the chemical composition is mass%, C: 0.020 to 0.100%, Si: 3.30 to 3.75%, Mn: 0.010 to 0.300%, S and / or Se: 0.001 to 0.050% in total, sol. Al: 0.010 to 0.065%, N: 0.002 to 0.015%, Sn: 0 to 0.500%, Cr: 0 to 0.500%, Cu: 0 to 0.500%, and , and the balance: Fe and impurities are hot-rolled to produce a steel plate.
In the cold rolling process, the steel sheet after the hot rolling process is cold rolled one or more times.
In the annealing step before final cold rolling, the steel sheet before the final cold rolling is annealed in one or a plurality of cold rollings.
The decarburization annealing step includes a temperature raising step and a decarburization step. In the temperature raising step, the steel sheet is heated to the decarburization annealing temperature, and until the temperature of the steel sheet reaches at least 500 to 700 ° C., A steel plate is heated at an average temperature increase rate of 800 to 2400° C./sec. In the decarburization step, the steel sheet after the heating step is held at a decarburization annealing temperature of 800 to 950°C.
In the annealing separator application step, an annealing separator is applied to the surface of the steel sheet after the decarburization annealing step.
In the finish annealing step, finish annealing is performed on the steel sheet to which the annealing separator is applied.

The method for producing a grain-oriented electrical steel sheet according to the present disclosure can suppress the occurrence of magnetic aging even when the Si content is 3.30% or more, and can produce a grain-oriented electrical steel sheet that provides excellent iron loss properties.

FIG. 1 is a flowchart showing manufacturing steps of a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment. FIG. 2 is a schematic diagram showing a heat pattern in the decarburization annealing step in FIG.

The present inventors investigated the reason why a grain-oriented electrical steel sheet having a Si content of 3.30% or more cannot provide a sufficient iron loss corresponding to the Si content.

By increasing the Si content in the grain-oriented electrical steel sheet as described above, the electrical resistance of the steel sheet can be increased and the eddy current loss can be reduced. As a result, iron loss characteristics are improved and iron loss is reduced.

However, as a result of investigation by the present inventor, the Si content is increased to 3.30% or more, the chemical composition is mass%, C: 0.020 to 0.100%, Si: 3.30 to 3.75% , Mn: 0.010 to 0.300%, S and/or Se: 0.001 to 0.050% in total, sol. Al: 0.010 to 0.065%, N: 0.002 to 0.015%, Sn: 0 to 0.500%, Cr: 0 to 0.500%, Cu: 0 to 0.500%, and , and the balance: Fe and impurities.

Therefore, the present inventors further investigated the cause of magnetic aging in a grain-oriented electrical steel sheet manufactured using a slab having the above chemical composition with an increased Si content of 3.30% or more. As a result, the present inventors found that when a grain-oriented electrical steel sheet is manufactured using a slab having the above chemical composition in which the Si content is increased to 3.30% or more, the following phenomenon occurs in the decarburization annealing process. I just found out that there is.

When producing a grain-oriented electrical steel sheet using a slab with the above chemical composition in which the Si content is increased to 3.30% or more, a large amount of SiO ₂ is generated on the surface layer of the steel sheet in the temperature rising process of the decarburization annealing process. , an outer oxide layer forms. The outer oxide layer formed during the temperature rise in the decarburization annealing process suppresses decarburization during the decarburization process. Therefore, C (carbon) remains excessively in the steel sheet after the decarburization annealing process. As a result, in the produced grain-oriented electrical steel sheet, carbides are formed in the steel sheet due to aging. This carbide lowers the iron loss properties of the grain-oriented electrical steel sheet, causing magnetic aging.

Based on the above knowledge, the present inventors investigated a method for suppressing the generation of SiO ₂ in the decarburization annealing process. The formation temperature range of SiO ₂ is 500-700°C. Hereinafter, this temperature range is referred to as "SiO ₂ generation temperature range". Since the decarburization annealing temperature is 800° C. or more and the decarburization annealing is performed in a moist nitrogen-hydrogen mixed atmosphere, SiO ₂ is difficult to form during the decarburization annealing (during holding at the decarburization annealing temperature). Therefore, during the temperature rise in the decarburization annealing process, while the steel sheet temperature stays in the SiO ₂ production temperature range, SiO ₂ is produced and an outer oxide layer is formed.

Considering the above mechanism, the present inventors thought that the generation of SiO ₂ could be suppressed by shortening the residence time of the steel sheet in the SiO ₂ generation temperature range in the decarburization annealing process. As a result of further investigation, when using a slab having a chemical composition with a high Si content of 3.30% or more, in the decarburization annealing process, the average heating rate of the steel sheet in the SiO ₂ generation temperature range was 800 ° C/sec. With the above, it was found that even if the Si content is high, the formation of an outer oxide layer can be sufficiently suppressed, sufficient decarburization is possible, and excellent iron loss properties can be obtained.

The method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment, which has been completed based on the above findings, includes a hot rolling process, a cold rolling process, an annealing process before final cold rolling, a decarburization annealing process, and an annealing separator application. and a finish annealing step.
In the hot rolling process, the chemical composition is mass%, C: 0.020 to 0.100%, Si: 3.30 to 3.75%, Mn: 0.010 to 0.300%, S and / or Se: 0.001 to 0.050% in total, sol. Al: 0.010 to 0.065%, N: 0.002 to 0.015%, Sn: 0 to 0.500%, Cr: 0 to 0.500%, Cu: 0 to 0.500%, and , and the balance: Fe and impurities are hot-rolled to produce a steel plate.
In the cold rolling process, the steel sheet after the hot rolling process is cold rolled one or more times.
In the annealing step before final cold rolling, the steel sheet before the final cold rolling is annealed in one or a plurality of cold rollings.
The decarburization annealing step includes a temperature raising step and a decarburization step. In the temperature raising step, the steel sheet is heated to the decarburization annealing temperature, and until the temperature of the steel sheet reaches at least 500 to 700 ° C., A steel plate is heated at an average temperature increase rate of 800 to 2400° C./sec. In the decarburization step, the steel sheet after the heating step is held at a decarburization annealing temperature of 800 to 950°C.
In the annealing separator application step, an annealing separator is applied to the surface of the steel sheet after the decarburization annealing step.
In the finish annealing step, finish annealing is performed on the steel sheet to which the annealing separator is applied.

The chemical composition of the slab is selected from the group consisting of Sn: 0.010 to 0.500%, Cr: 0.010 to 0.500%, and Cu: 0.010 to 0.500%1 It may contain more than one seed.

In the temperature raising step of the decarburization annealing step in the manufacturing process, the oxygen potential in the atmosphere may be set to 0.001 or less until the temperature of the steel sheet reaches at least 500 to 700°C.

The thickness of the grain-oriented electrical steel sheet manufactured by the manufacturing process described above may be 0.18 to 0.23 mm. A preferable plate thickness is 0.20 mm or less.

Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment will be described in detail. In addition, in this specification, % regarding the content of an element means mass %, unless otherwise specified.

[Manufacturing process flow]
FIG. 1 is a flowchart of a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment. Referring to FIG. 1, this manufacturing method comprises a hot rolling step (S1) in which hot rolling is performed on a slab, and one or more times of hot rolling on a steel plate (hot rolled steel plate) after hot rolling. A cold rolling step (S2) that performs cold rolling (S20) and a final cold rolling that performs annealing treatment on the steel plate before the final cold rolling among the cold rolling of one or two or more times A pre-rolling annealing step (S3), a decarburization annealing step (S4) in which decarburization annealing is performed on the steel sheet (cold-rolled steel sheet) after the cold rolling process, and annealing on the surface of the steel sheet after the deoxidizing annealing process. It includes an annealing separating agent application step (S5) of applying a separating agent, and a finish annealing step (S6) of performing finish annealing on the steel sheet to which the annealing separating agent has been applied. Each step S1 to S6 will be described below.

[Hot rolling step (S1)]
In the hot rolling step (S1), the prepared slab is hot rolled to manufacture a hot rolled steel sheet. The chemical composition of the slab contains the following elements:

[Essential elements in the chemical composition of the slab]
C: 0.020-0.100%
Carbon (C) is effective for structure control during the manufacturing process until the decarburization annealing process is completed. However, if the C content is less than 0.02%, the above effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.100%, decarburization becomes insufficient and magnetic aging occurs even if the decarburization annealing step described later is performed. In this case, sufficient iron loss characteristics cannot be obtained. Therefore, the C content is 0.020-0.100%. A preferred lower limit for the C content is 0.030%, more preferably 0.040%. A preferable upper limit of the C content is 0.090%, more preferably 0.080%.

Si: 3.30-3.75%
Silicon (Si) increases the electrical resistance of the grain-oriented electrical steel sheet and reduces eddy current loss among core losses. If the Si content is less than 3.30%, the above effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 3.75%, the cold workability of the steel deteriorates. Therefore, the Si content is 3.30-3.75%. A preferable lower limit of the Si content is 3.35%, more preferably 3.40%. A preferable upper limit of the Si content is 3.70%, more preferably 3.60%.

Mn: 0.010-0.300%
Manganese (Mn) increases the specific resistance of grain-oriented electrical steel sheets to reduce core loss. Mn further enhances hot workability and suppresses the occurrence of cracks during hot rolling. Mn further combines with S and/or Se to form fine MnS and/or fine MnSe in the annealing step prior to final cold rolling. Fine MnS and fine MnSe serve as precipitation nuclei for fine AlN that is utilized as an inhibitor. Therefore, in the annealing step before the final cold rolling, if the amount of precipitated fine MnS and fine MnSe is large, a sufficient amount of fine AlN can be obtained. If the Mn content is less than 0.010%, a sufficient amount of fine MnS and fine MnSe will not precipitate. On the other hand, if the Mn content exceeds 0.300%, the magnetic flux density of the grain-oriented electrical steel sheet decreases. Therefore, the Mn content is 0.010-0.300%. A preferred lower limit for the Mn content is 0.020%, more preferably 0.030%. A preferable upper limit of the Mn content is 0.200%, more preferably 0.150%.

S and / or Se: 0.001 to 0.050% in total
Sulfur (S) and selenium (Se) combine with Mn during the manufacturing process to form fine MnS and/or fine MnSe described above. Fine MnS and fine MnSe serve as precipitation nuclei for fine AlN that is utilized as an inhibitor. Therefore, in the annealing step before the final cold rolling, if the amount of precipitated fine MnS and fine MnSe is large, a sufficient amount of fine AlN can be obtained. If the total content of S and/or Se is less than 0.001%, sufficient amounts of fine MnS and fine MnSe cannot be obtained. On the other hand, if the total content of S and/or Se exceeds 0.050%, MnS and/or MnSe may remain even in the steel sheet after the finish annealing process. In this case, the magnetic properties are degraded. Therefore, the total content of S and/or Se is 0.001-0.050%. A preferred lower limit for the total content of S and/or Se is 0.005%. A preferable upper limit of the total content of S and/or Se is 0.040%, more preferably 0.030%.

sol. Al: 0.010-0.065%
Aluminum (Al) combines with N to form AlN and functions as an inhibitor during the manufacturing process of the grain-oriented electrical steel sheet. sol. If the Al content is less than 0.010%, a sufficient amount of AlN to function as an inhibitor cannot be obtained. On the other hand, sol. If the Al content exceeds 0.065%, AlN coarsens and the inhibitor strength decreases. Therefore, sol. The Al content is 0.010-0.065%. sol. A preferable lower limit of the Al content is 0.015%, more preferably 0.020%. sol. A preferable upper limit of the Al content is 0.055%, more preferably 0.045%. In this specification, sol. Al means acid-soluble Al. Therefore, sol. The Al content is the content of acid-soluble Al.

N: 0.002-0.015%
Nitrogen (N) combines with Al to form AlN and functions as an inhibitor during the manufacturing process of the grain-oriented electrical steel sheet. In order to reduce the N content to less than 0.002%, excessive refining is required in the steelmaking process, which increases production costs. Therefore, the lower limit of the N content is 0.002%. On the other hand, if the N content in the steel material exceeds 0.015%, a large number of blisters (voids) are likely to form in the steel sheet during cold rolling. Therefore, the N content is 0.002-0.015%. A preferable lower limit of the N content is 0.004%, more preferably 0.006%. A preferable upper limit of the N content is 0.012%, more preferably 0.0010%.

The balance of the chemical composition of the slab according to this embodiment consists of Fe and impurities. Here, impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when slabs, which are materials of grain-oriented electrical steel sheets, are industrially manufactured. It means a permissible range that does not adversely affect the grain-oriented electrical steel sheet manufactured by the manufacturing method.

[Arbitrary element in chemical composition of slab]
The chemical composition of the slab described above may contain one or more selected from the group consisting of Sn, Cr and Cu in place of part of Fe.

Sn: 0-0.500%
Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When included, Sn improves the properties of the oxide layer formed during the decarburization annealing process, and also improves the properties of the primary coating formed using this oxide layer during the final annealing process. Further, Sn stabilizes the formation of the oxide layer and the primary coating, thereby improving the magnetic properties of the grain-oriented electrical steel sheet and suppressing variations in the magnetic properties. Sn is also a grain boundary segregation element and stabilizes secondary recrystallization. However, if the Sn content exceeds 0.500%, the surface of the steel sheet becomes difficult to oxidize, and the formation of the primary coating may become insufficient. Therefore, the Sn content is 0-0.500%. A preferable lower limit of the Sn content for obtaining the above effect more effectively is 0.010%, more preferably 0.020%. A preferable upper limit of the Sn content is 0.300%, more preferably 0.200%.

Cr: 0-0.500%
Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When included, Cr improves the properties of the oxide layer formed during the decarburization annealing process, and also improves the properties of the primary coating formed using this oxide layer during the final annealing process. Furthermore, Cr stabilizes the formation of the oxide layer and the primary coating, thereby improving the magnetic properties of the grain-oriented electrical steel sheet and suppressing variations in the magnetic properties. However, if the Cr content exceeds 0.500%, the formation of the primary coating may become unstable. Therefore, the Cr content is 0-0.500%. A preferable lower limit of the Cr content for obtaining the above effect more effectively is 0.010%, more preferably 0.020%. A preferable upper limit of the Cr content is 0.200%, more preferably 0.150%.

Cu: 0-0.500%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu promotes the precipitation of fine MnS that serves as nuclei for the formation of AlN. However, if the Cu content is too high, CuS precipitates are precipitated, and the CuS precipitates may remain even after final annealing. If CuS precipitates remain in the steel, the magnetic properties of the grain-oriented electrical steel sheet deteriorate. Therefore, the Cu content is 0-0.500%. A preferable lower limit of the Cu content is 0.010%, more preferably 0.030%, and still more preferably 0.050%. A preferable upper limit of the Cu content is 0.400%, more preferably 0.300%.

[Method for producing slab having the above chemical composition]
An example of a method for manufacturing a slab having the above chemical composition is as follows. Molten steel having the above chemical composition is produced (smelted). Using molten steel, a slab is manufactured by a continuous casting method.

[Hot rolling step (S1) using the above slab]
The prepared slab having the above chemical composition is hot-rolled using a hot rolling mill to produce a steel sheet (hot-rolled steel sheet). First, the steel material is heated. For example, the slab is loaded into a known heating furnace or a known soaking furnace and heated. The preferred heating temperature for the slab is 1300-1400°C, more preferably 1320-1380°C.

The heated slab is hot rolled using a hot rolling mill to produce a steel plate (hot rolled steel plate). The hot rolling mill comprises a roughing mill and a finishing mill arranged downstream of the roughing mill. A roughing mill comprises one or more roughing stands in a row. Each roughing stand includes a plurality of rolls arranged one above the other. The finishing mill likewise comprises a row of finishing stands. Each finishing stand includes a plurality of rolls arranged one above the other. After the heated slab is rolled by a rough rolling mill, it is further rolled by a finishing rolling mill to produce a hot-rolled steel sheet.

The thickness of the hot-rolled steel sheet manufactured by hot rolling is not particularly limited, and may be a known thickness. The finishing temperature in the hot rolling process (the temperature of the steel sheet at the delivery side of the finishing rolling stand where the steel sheet is finally reduced in the finishing mill) is, for example, 1000 to 1100.degree. The finishing temperature is the surface temperature (° C.) of the steel sheet obtained by a thermometer placed on the delivery side of the finishing rolling stand that performs the final reduction. A steel plate is manufactured by the rolling process described above.

[Cold rolling step (S2)]
In the cold rolling step (S2), the manufactured steel sheet is cold rolled one or more times. Cold rolling is performed using a cold rolling mill. The cold rolling mill may be a tandem rolling mill comprising a plurality of cold rolling stands arranged in a row, or may be a reverse rolling mill comprising a single cold rolling stand. . Each cold rolling stand includes multiple cold rolling rolls.

As shown in FIG. 1, in the cold rolling step, cold rolling may be performed only once (S20), or cold rolling (S20) may be performed multiple times. When cold rolling is performed multiple times, intermediate annealing may be performed for the purpose of softening the steel sheet after cold rolling is performed using the cold rolling mill. In this case, the subsequent cold rolling is carried out after the intermediate annealing treatment. That is, an intermediate annealing treatment may be performed during cold rolling.

Known conditions are sufficient for the conditions of the intermediate annealing treatment performed between the cold rolling and the subsequent cold rolling. The annealing temperature in the intermediate annealing treatment is, for example, 900-1200° C., and the holding time at the annealing temperature is 30-180 seconds. After reducing the strain introduced into the steel sheet in the previous cold rolling (softening the steel sheet) by the intermediate annealing treatment, the next cold rolling is performed.

In addition, when performing a plurality of cold rolling steps without performing an intermediate annealing step, it may be difficult to obtain uniform properties in the produced grain-oriented electrical steel sheet. On the other hand, when cold rolling is performed a plurality of times and intermediate annealing is performed between each cold rolling, the produced grain-oriented electrical steel sheet may have a low magnetic flux density. Therefore, the number of times of cold rolling and the presence or absence of intermediate annealing treatment are determined according to the properties and manufacturing costs required for the grain-oriented electrical steel sheet to be finally manufactured.

In addition, in the cold rolling step, as described above, only one cold rolling may be performed.

A preferred cumulative cold rolling reduction in the single or multiple cold rolling is 80 to 95%. Here, the cumulative cold rolling rate (%) is defined as follows.
Cold rolling rate (%) = 100 - thickness of cold-rolled steel sheet after final cold rolling/thickness of steel sheet before the start of first cold rolling x 100

In addition, in the cold rolling process, when only one cold rolling is performed, the above cold rolling reduction is the cold rolling reduction in only one cold rolling. A steel sheet manufactured by a cold rolling process is wound into a coil.

[Annealing step before final cold rolling (S3)]
In the annealing step before final cold rolling (S3), the steel plate before the final cold rolling (S20) is subjected to final Annealing treatment before cold rolling is performed. The conditions for the annealing treatment before final cold rolling are the same as the conditions for the intermediate annealing treatment described above. The annealing temperature in the annealing step before final cold rolling is, for example, 900 to 1200° C., and the holding time at the annealing temperature is 30 to 180 seconds.

[Decarburization annealing step (S4)]
In the decarburization annealing step (S4), the steel sheet (cold-rolled steel sheet) after the cold rolling step (S2) is subjected to decarburization annealing to develop primary recrystallization.

FIG. 2 is a schematic diagram showing a heat pattern in the decarburization annealing step (S4). Referring to FIG. 2, the decarburization annealing step (S4) includes a heating step (S41), a decarburization step (S42), and a cooling step (S43). In the temperature raising step (S41), the steel sheet is heated to the decarburization annealing temperature Ta. In the decarburization step (S42), the steel sheet heated to the decarburization annealing temperature Ta is subjected to decarburization annealing to develop primary recrystallization. In the cooling step (S43), the steel sheet after the decarburization step (S42) is cooled by a known method. In the present embodiment, the Si content in the steel sheet is 3.30% or more by significantly increasing the temperature increase rate in the SiO ₂ generation temperature range (500 to 700° C.) in the temperature increase step (S41). maintains excellent decarburization properties. Details of each step will be described below.

[Temperature raising step (S41)]
In the temperature raising step (S41), first, the steel plate after the cold rolling step is charged into a heat treatment furnace. In the heat treatment furnace for decarburization annealing according to the present embodiment, the temperature of the cold-rolled steel sheet is raised to the decarburization annealing temperature by, for example, high-frequency induction heating. The manufacturing conditions in the heating step are as follows.

Average heating rate RR _500-700 : 800 to 2400 ° C./sec In the heating process, the average heating rate from 500 ° C. to 700 ° C. of the steel sheet is the average heating rate RR _500-700 . (°C/sec). As described above, the Si content in the chemical composition of the slab of this embodiment is as high as 3.30% or more. In this case, in the heating step, the formation of SiO ₂ on the surface layer of the steel sheet is promoted, and the coverage with the outer oxide layer increases. The outer oxide layer suppresses decarburization of the steel sheet. As a result, decarburization becomes insufficient, and magnetic aging occurs in the produced grain-oriented electrical steel sheet. As a result, excellent iron loss properties cannot be obtained.

SiO ₂ is significantly generated in the temperature range of 500 to 700° C. (SiO ₂ generation temperature range). The decarburization annealing temperature is 800° C. or higher as described later. Therefore, the _SiO2 formation temperature range is lower than the decarburization annealing temperature. In other words, the steel sheet passes through the SiO ₂ generation temperature range in the heating process for heating the steel sheet. If the residence time of the steel sheet in the SiO ₂ generation temperature range becomes longer, the generation of SiO ₂ in the surface layer of the steel sheet is promoted. Therefore, in the present embodiment, the residence time of the steel sheet in the _SiO2 generation temperature range is shortened by increasing the heating rate in the _SiO2 generation temperature range.

If the average temperature increase rate RR _500-700 is 800° C./second or more, the residence time of the cold-rolled steel sheet in the SiO ₂ formation temperature range can be significantly shortened. Therefore, even if the Si content in the chemical composition of the slab is as high as 3.30% or more, it is possible to suppress the formation of an outer oxide layer and suppress the deterioration of decarburization. As a result, excellent core loss properties are obtained in the grain-oriented electrical steel sheet.

If the average heating rate RR _500-700 exceeds 2400° C./sec, grains with corresponding orientations that promote the growth of grains with Goss orientation in the final annealing process will decrease. In this case, the growth of Goss-oriented grains is suppressed in the finish annealing step. As a result, the Goss orientation density (B8/Bs) decreases, and the hysteresis loss among the core losses increases. As a result, excellent iron loss properties cannot be obtained. In addition, since the growth of Goss-oriented grains is suppressed, the magnetic flux density also decreases. Therefore, the upper limit of the average heating rate RR _500-700 is 2400° C./sec.

A preferable lower limit of the average heating rate RR _500-700 is 900° C./second, more preferably 1000° C./second. The preferred upper limit of the average heating rate RR _500-700 is 2100°C/sec.

The atmosphere during the heating process is a dry nitrogen atmosphere with an oxygen potential (P _H2O /P _H2 ) of 0.1 or less. Preferably, the oxygen potential (P _H2O /P _H2 ) of the atmosphere at least in the SiO ₂ formation temperature range during the temperature raising step is set to 0.001 or less. In this case, it is possible to further suppress the generation of SiO ₂ in the surface layer of the steel sheet in the heating step. As a result, even better core loss properties are obtained.

Average heating rate RR _500-700 is measured by the following method. A plurality of thermometers are installed in the heat treatment furnace for measuring the surface temperature of the steel sheet. A plurality of thermometers are arranged from upstream to downstream of the heat treatment furnace. Based on the temperature of the steel sheet measured by the thermometer and the time required for the steel sheet temperature to rise from 500°C to 700°C, an average heating rate RR _500-700 is obtained.

[Decarburization step (S42)]
In the decarburization step (S42) in the decarburization annealing step (S4), decarburization annealing is performed by holding the steel sheet after the temperature raising step (S41) at the decarburization annealing temperature Ta. This causes the steel sheet to exhibit primary recrystallization. The atmosphere during the decarburization step may be a well-known atmosphere, such as a wet nitrogen-hydrogen mixed atmosphere containing hydrogen and nitrogen. By performing decarburization annealing, carbon in the steel sheet is removed from the steel sheet, and primary recrystallization occurs. The manufacturing conditions in the decarburization process are as follows.

Decarburization annealing temperature Ta: 800-950°C
The decarburization annealing temperature Ta, as described above, corresponds to the furnace temperature of the heat treatment furnace in which decarburization annealing is performed, and corresponds to the temperature of the steel sheet during decarburization annealing. If the decarburization annealing temperature Ta is less than 800°C, the crystal grains of the steel sheet after primary recrystallization are too small. In this case, the secondary recrystallization does not sufficiently occur in the finish annealing step (S6). On the other hand, if the decarburization annealing temperature Ta exceeds 950° C., the crystal grains of the steel sheet after primary recrystallization are too large. Also in this case, the secondary recrystallization does not sufficiently occur in the finish annealing step (S6). If the decarburization annealing temperature Ta is 800 to 950° C., the crystal grains of the steel sheet after primary recrystallization will have an appropriate size, and secondary recrystallization will sufficiently occur in the finish annealing step (S6).

The holding time at the decarburization annealing temperature Ta in the decarburization step (S42) is not particularly limited. The holding time at the decarburization annealing temperature Ta is, for example, 15 to 150 seconds.

[Cooling step (S43)]
In the cooling step (S43), the steel sheet after the decarburization step (S42) is cooled to normal temperature by a well-known method. The cooling method may be air cooling or water cooling. Preferably, the steel sheet after the decarburization step is allowed to cool. In the decarburization annealing step (S4), the steel sheet is subjected to the decarburization annealing treatment through the above steps.

[Annealing separating agent application step (S5)]
An annealing separator application step (S5) is performed on the steel sheet after the decarburization annealing step (S4). In the annealing separator application step (S5), an annealing separator is applied to the surface of the steel sheet. Specifically, an aqueous slurry containing an annealing separator is applied to the surface of the steel sheet. The aqueous slurry is prepared by adding water to the annealing separator and stirring. The annealing separator contains magnesium oxide (MgO). Preferably, MgO is the main component of the annealing separator. Here, the "main component" means that the content of MgO in the annealing separator is 60.0% or more by mass. The annealing separator may contain well-known additives in addition to MgO.

In the annealing separator application step, an aqueous slurry of an annealing separator is applied onto the surface of the steel sheet. A steel sheet coated with an annealing separator on its surface is wound up into a coil. After coiling the steel sheet, the finish annealing step (S6) is performed.

After coating the surface of the steel sheet with an annealing separating agent in the form of aqueous slurry and forming the steel sheet into a coil shape, the baking treatment may be performed before performing the finish annealing step. In the baking treatment, a coiled steel plate is put into a furnace maintained at 400 to 1000° C. and held there (baking treatment). This dries the applied annealing separator. The retention time is, for example, 10-90 seconds.

The finish annealing process may be performed on the coiled steel sheet to which the annealing separator is applied without performing the baking treatment.

[Finish annealing step (S6)]
After the annealing separator application step (S5), the steel sheet is subjected to the finish annealing step (S6) to develop secondary recrystallization. A finish annealing process is implemented using a heat treatment furnace. Manufacturing conditions in the finish annealing step are, for example, as follows. In addition, the atmosphere in the furnace in the finish annealing is a well-known atmosphere.

Finish annealing temperature: 1150-1250°C
Holding time at finish annealing temperature: 5 to 30 hours If the finish annealing temperature is less than 1150°C, sufficient secondary recrystallization does not occur, and the purification to remove precipitates used for secondary recrystallization is sufficient. is not. As a result, the produced grain-oriented electrical steel sheet has low magnetic properties. On the other hand, even if the finish annealing temperature exceeds 1250° C., the effects on secondary recrystallization and purification are low, and problems such as deformation of the steel sheet occur. If the finish annealing temperature is 1150 to 1250° C., assuming that the holding time is appropriate, sufficient secondary recrystallization occurs and the magnetic properties are enhanced. Furthermore, a primary coating containing forsterite is formed soundly on the surface of the steel sheet.

In addition, each element of the chemical composition of the steel sheet is removed to some extent from the components in the steel by the finish annealing step (S6). In particular, S, Al, N, etc., which function as inhibitors, are largely removed.

The grain-oriented electrical steel sheet according to the present embodiment is manufactured by the above-described manufacturing process. In the produced grain-oriented electrical steel sheet, although the Si content is as high as 3.30% or more, sufficient decarburization is possible in the decarburization annealing step. Therefore, magnetic aging is unlikely to occur, and excellent iron loss properties can be obtained.

A primary coating containing forsterite is formed on the surface of the grain-oriented electrical steel sheet after the finish annealing process.

[Secondary film forming step]
In the method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment, a secondary coating forming step may be further carried out after the finish annealing step (S6), if necessary. In the secondary coating forming step, the surface (on the primary coating) of the grain-oriented electrical steel sheet after cooling in the final annealing step is coated with an insulating coating agent mainly composed of colloidal silica and phosphate, and then baked. . As a result, a secondary coating, which is a tension insulating coating, is formed on the primary coating.

[Magnetic domain refining process]
The grain-oriented electrical steel sheet according to the present embodiment may further be subjected to a magnetic domain refining treatment step after the finish annealing step or the secondary coating forming step, if necessary. In the magnetic domain refining treatment step, the surface of the grain-oriented electrical steel sheet is irradiated with a laser beam having a magnetic domain refining effect, or grooves are formed on the surface. In this case, a grain-oriented electrical steel sheet with even better magnetic properties can be produced.

[EXAMPLES] Below, an Example demonstrates the aspect of this invention concretely. These examples are examples for confirming the effect of the present invention, and do not limit the present invention.

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition is mass %, C: 0.075%, Mn: 0.075%, S: 0.028%, sol. A slab containing Al: 0.028%, N: 0.008%, Si in the content (% by mass) shown in Table 1, and the balance being Fe and impurities was prepared.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

The hot rolled steel sheet was subjected to an annealing step prior to final cold rolling. In the annealing step before the final cold rolling, the hot-rolled steel sheet was heated to 1120° C., and then the steel sheet was annealed at an annealing temperature of 900° C. and a holding time at the annealing temperature of 40 seconds.

The steel sheet after the annealing process before final cold rolling was cold rolled once (final cold rolling) to produce a cold-rolled steel sheet with a thickness of 0.23 mm. A decarburization annealing process was performed on the cold-rolled steel sheet after the cold rolling process. In the decarburization annealing step, the decarburization annealing temperature was set to 830°C. The atmosphere in the heat treatment furnace for carrying out the decarburization annealing treatment was a well-known wet nitrogen-hydrogen mixed atmosphere containing hydrogen and nitrogen. Then, the oxygen potential (P _H2O /P _H2 ) in the atmosphere in the heating process was set to 0.1. Furthermore, the values shown in Table 1 were taken as the average temperature increase rate RR _500-700 in the SiO ₂ temperature range (500-700° C.) during temperature increase.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary coating forming step, the surface (on the primary coating) of the grain-oriented electrical steel sheet after the finish annealing step was coated with an insulating coating mainly composed of colloidal silica and phosphate, and then baked. As a result, a secondary coating, which was a tension insulating coating, was formed on the primary coating. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
[Magnetic property evaluation test]
The magnetic properties (magnetic flux density B8 and core loss W _17/50 ) of the grain-oriented electrical steel sheets of each test number were evaluated by the following method in accordance with JIS C2556:2015. Specifically, a magnetic field of 800 A/m was applied to each sample, and the magnetic flux density B8(T) was measured. Also, iron loss W _17/50 (W/kg) was measured at a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T. Also, the saturation magnetic flux density Bs(T) was determined by the following formula using the Si content (% by mass).
Bs = 2.2032 - 0.0581 Si
Based on the obtained magnetic flux density B8 and saturation magnetic flux density Bs, the Goss orientation integration degree (B8/Bs), which is the ratio of the magnetic flux density B8 to the saturation magnetic flux density Bs, was obtained.

Table 1 shows the obtained magnetic flux density B8, Goss orientation density B8/Bs, and iron loss _W17 /50.

[Magnetic aging test]
After measuring the magnetic flux density B8 and the core loss W _17/50 by the above method, aging heat treatment was performed with a dry nitrogen atmosphere, a heat treatment temperature of 150° C., and a holding time at the heat treatment temperature of 120 hours. The iron loss W _17/50 (W/kg) was determined for the sample after the aging heat treatment by the method described above. Then, the iron loss difference ΔW (%) before the aging heat treatment and after the aging heat treatment was obtained from the following formula.
ΔW={(iron loss W _17/50 after aging heat treatment−iron loss W _17/50 before aging heat treatment)/iron loss W _17/50 before aging heat treatment}×100

When the iron loss difference ΔW was 5% or less, it was judged that magnetic aging was suppressed and excellent iron loss characteristics were obtained (“◯” in Table 1). On the other hand, when the iron loss difference ΔW exceeded 5%, it was determined that magnetic aging occurred and the iron loss characteristics were lowered (“X” in Table 1).

[Test results]
Table 1 shows the test results obtained. With reference to Table 1, in test numbers 10 to 13, 18 to 21, 26 to 29, and 34 to 37, the chemical composition of the slab was appropriate, and the conditions in each manufacturing process were also appropriate. . As a result, in any test number, the magnetic flux density B8 was as high as 1.920 T or more, and the Goss direction integration degree B8/Bs was as high as 0.958 or more. Furthermore, the iron loss W _17/50 was as low as 0.812 W/kg or less. Further, the iron loss difference ΔW obtained by the magnetic aging test was all 5% or less, indicating that the magnetic aging was sufficiently suppressed and excellent iron loss characteristics were obtained.

On the other hand, in test numbers 1 to 6, the Si content was too low. Therefore, at least the iron loss W _17/50 exceeded 0.812 W/kg, and the iron loss characteristics were low.

On the other hand, in test numbers 7 to 9, 15 to 17, 23 to 25, and 31 to 33, the chemical composition of the slabs was appropriate, but the average heating rate RR _500-700 was too low. As a result, the magnetic flux density B8 was less than 1.920 T, and the Goss orientation density B8/Bs was less than 0.958. Furthermore, the core loss W _17/50 was also high, exceeding 0.812 W/kg. Furthermore, the iron loss difference ΔW obtained by the magnetic aging test exceeded 5%, and magnetic aging occurred to lower the iron loss characteristics.

In test numbers 14, 22, 30, and 38, the chemical composition of the slabs was appropriate, but the average heating rate RR _500-700 exceeded 2400°C/sec, which was too high. Therefore, although the iron loss difference ΔW obtained by the magnetic aging test was 5% or less, the magnetic flux density B8 was less than 1.920 T, the Goss orientation accumulation degree B8/Bs was less than 0.958, and the iron loss W _17/50 also exceeded 0.812 W/kg.

In test numbers 39-44, the Si content was too high. Therefore, in test numbers 39 to 42, cracks occurred in the steel sheets during cold rolling. Further, in test numbers 43 and 44, the magnetic flux density B8 was less than 1.920 T, and the Goss direction integration degree B8/Bs was less than 0.958. Furthermore, the core loss W _17/50 was also high, exceeding 0.812 W/kg. Furthermore, the iron loss difference ΔW obtained by the magnetic aging test exceeded 5%, and magnetic aging occurred to lower the iron loss characteristics.

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition is mass %, C: 0.075%, Mn: 0.075%, S: 0.028%, sol. Al: 0.028%, N: 0.008%, Sn: 0.100%, Cu: 0.070%, Cr: 0.050%, and the contents shown in Table 2 (% by mass) A slab was prepared containing about 10% of Si, with the balance being Fe and impurities.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

The hot rolled steel sheet was subjected to an annealing step prior to final cold rolling. In the annealing step before final cold rolling, the hot-rolled steel sheet was heated to 1120° C., and then annealed at an annealing temperature of 900° C. for 40 seconds.

The steel sheet after the annealing process before final cold rolling was cold rolled once (final cold rolling) to produce a cold-rolled steel sheet with a thickness of 0.23 mm. A decarburization annealing process was performed on the cold-rolled steel sheet after the cold rolling process. In the decarburization annealing step, the decarburization annealing temperature was set to 830°C. The atmosphere in the heat treatment furnace for carrying out the decarburization annealing treatment was a well-known wet nitrogen-hydrogen mixed atmosphere containing hydrogen and nitrogen. Then, the oxygen potential (P _H2O /P _H2 ) in the atmosphere in the heating process was set to 0.1. Furthermore, the values shown in Table 2 were taken as the average temperature increase rate RR _500-700 in the SiO ₂ temperature range (500-700° C.) during temperature increase.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary coating forming step, the surface (on the primary coating) of the grain-oriented electrical steel sheet after the finish annealing step was coated with an insulating coating mainly composed of colloidal silica and phosphate, and then baked. As a result, a secondary coating, which was a tension insulating coating, was formed on the primary coating. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
By the same method as in Example 1, the magnetic flux density B8, the Goss orientation density B8/Bs, the iron loss _W17 /50, and the iron loss difference ΔW of each test number were obtained.

[Test results]
Table 2 shows the test results obtained. With reference to Table 2, in test numbers 10 to 13, 18 to 21, 26 to 29, and 34 to 37, the chemical composition of the slab was appropriate, and the conditions in each manufacturing process were also appropriate. . As a result, in any test number, the magnetic flux density B8 was as high as 1.923 T or more, and the Goss direction integration degree B8/Bs was as high as 0.961 or more. Furthermore, the iron loss W _17/50 was as low as 0.795 W/kg or less. Further, the iron loss difference ΔW obtained by the magnetic aging test was all 5% or less, indicating that the magnetic aging was sufficiently suppressed and excellent iron loss characteristics were obtained.

On the other hand, in test numbers 1 to 6, the Si content was too low. Therefore, at least the iron loss W _17/50 exceeded 0.795 W/kg, and the iron loss characteristics were low.

On the other hand, in test numbers 7 to 9, 15 to 17, 23 to 25, and 31 to 33, the chemical composition of the slabs was appropriate, but the average heating rate RR _500-700 was too low. As a result, the magnetic flux density B8 was less than 1.923 T, and the Goss orientation density B8/Bs was less than 0.961. Furthermore, the iron loss W _17/50 was also high, exceeding 0.795 W/kg. Furthermore, the iron loss difference ΔW obtained by the magnetic aging test exceeded 5%, and magnetic aging occurred to lower the iron loss characteristics.

In test numbers 14, 22, 30, and 38, the chemical composition of the slabs was appropriate, but the average heating rate RR _500-700 exceeded 2400°C/sec, which was too high. Therefore, although the iron loss difference ΔW obtained by the magnetic aging test was 5% or less, the magnetic flux density B8 was less than 1.923 T, the Goss orientation accumulation degree B8/Bs was less than 0.961, and the iron loss W _17/50 also exceeded 0.795 W/kg.

In test numbers 39-44, the Si content was too high. Therefore, in test numbers 39, 41 and 42, cracks occurred in the steel sheets during cold rolling. Further, in test numbers 40, 43 and 44, the magnetic flux density B8 was less than 1.923 T, and the Goss direction integration degree B8/Bs was less than 0.961. Furthermore, the iron loss W _17/50 was also high, exceeding 0.795 W/kg. Furthermore, the iron loss difference ΔW obtained by the magnetic aging test exceeded 5%, and magnetic aging occurred to lower the iron loss characteristics.

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition is mass %, C: 0.070%, Mn: 0.075%, S: 0.027%, sol. Al: 0.026%, N: 0.008%, Sn: 0.110%, Cu: 0.070%, Cr: 0.035%, and the contents shown in Table 3 (% by mass) A slab was prepared containing about 10% of Si, with the balance being Fe and impurities.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

The hot rolled steel sheet was subjected to an annealing step prior to final cold rolling. In the annealing step before final cold rolling, the hot-rolled steel sheet was heated to 1120° C., and then annealed at an annealing temperature of 900° C. for 40 seconds.

The steel sheet after the annealing process before final cold rolling was cold rolled once (final cold rolling) to produce a cold-rolled steel sheet with a thickness of 0.23 mm. A decarburization annealing process was performed on the cold-rolled steel sheet after the cold rolling process. In the decarburization annealing step, the decarburization annealing temperature was set to 830°C. The atmosphere in the heat treatment furnace for carrying out the decarburization annealing treatment was a well-known wet nitrogen-hydrogen mixed atmosphere containing hydrogen and nitrogen. Table 3 shows the oxygen potential (P _H2O /P _H2 ) in the atmosphere during the heating process. Furthermore, the values shown in Table 3 were taken as the average temperature increase rate RR _500-700 in the SiO ₂ temperature range (500-700° C.) during temperature increase.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary coating forming step, the surface (on the primary coating) of the grain-oriented electrical steel sheet after the finish annealing step was coated with an insulating coating mainly composed of colloidal silica and phosphate, and then baked. As a result, a secondary coating, which was a tension insulating coating, was formed on the primary coating. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
By the same method as in Example 1, the magnetic flux density B8, the Goss orientation density B8/Bs, the iron loss W17/50, and the iron loss difference ΔW for each test number were determined. In the magnetic annealing test, when the iron loss difference ΔW was 1% or less, it was determined that the magnetic aging was remarkably suppressed and that extremely excellent iron loss characteristics were obtained (“⊚” in Table 3). Moreover, when the iron loss difference ΔW was 5% or less, it was judged that magnetic aging was suppressed and excellent iron loss characteristics were obtained (“◯” in Table 3). On the other hand, when the iron loss difference ΔW exceeded 5%, it was determined that magnetic aging occurred and the iron loss characteristics were lowered (“X” in Table 3).

[Test results]
Table 3 shows the test results obtained. With reference to Table 3, in all of Test Nos. 1 to 32, the chemical composition of the slab was appropriate, and the conditions in each manufacturing process were also appropriate. As a result, in any test number, the magnetic flux density B8 was as high as 1.905 T or more, and the Goss direction integration degree B8/Bs was as high as 0.958 or more. Furthermore, the iron loss W _17/50 was as low as 0.821 W/kg or less. Further, the iron loss difference ΔW obtained by the magnetic aging test was all 5% or less, indicating that the magnetic aging was sufficiently suppressed and excellent iron loss characteristics were obtained.

Furthermore, in test numbers 17 to 32, the oxygen potential in the temperature rising step of the decarburization annealing step was 0.001 or less. Therefore, the iron loss difference ΔW obtained by the magnetic aging test was all 1% or less, indicating that the magnetic aging was sufficiently suppressed and extremely excellent iron loss characteristics were obtained.

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition is mass %, C: 0.075%, Mn: 0.075%, S: 0.027%, sol. Al: 0.026%, N: 0.008%, Sn: 0.100%, Cu: 0.070%, Cr: 0.035%, and the contents shown in Table 4 (% by mass) A slab was prepared containing about 10% of Si, with the balance being Fe and impurities.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

The hot rolled steel sheet was subjected to an annealing step prior to final cold rolling. In the annealing step before final cold rolling, the hot-rolled steel sheet was heated to 1120° C., and then annealed at an annealing temperature of 900° C. for 40 seconds.

The steel sheets after the annealing process before final cold rolling were cold rolled once (final cold rolling) to produce cold rolled steel sheets with a thickness of 0.20 mm or 0.18 mm. A decarburization annealing process was performed on the cold-rolled steel sheet after the cold rolling process. In the decarburization annealing step, the decarburization annealing temperature was set to 830°C. The atmosphere in the heat treatment furnace for carrying out the decarburization annealing treatment was a well-known wet nitrogen-hydrogen mixed atmosphere containing hydrogen and nitrogen. Then, the oxygen potential (P _H2O /P _H2 ) in the atmosphere in the heating process was set to 0.1. Further, the values shown in Table 4 were taken as the average temperature increase rate RR _500-700 in the SiO ₂ temperature range (500-700° C.) during temperature increase.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary coating forming step, the surface (on the primary coating) of the grain-oriented electrical steel sheet after the finish annealing step was coated with an insulating coating mainly composed of colloidal silica and phosphate, and then baked. As a result, a secondary coating, which was a tension insulating coating, was formed on the primary coating. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
By the same method as in Example 1, the magnetic flux density B8, the Goss orientation density B8/Bs, the iron loss W17/50, and the iron loss difference ΔW for each test number were obtained. In the magnetic annealing test, when the iron loss difference ΔW was 1% or less, it was judged that the magnetic aging was remarkably suppressed and extremely excellent iron loss characteristics were obtained (“⊚” in Table 4). Further, when the iron loss difference ΔW was 5% or less, it was judged that magnetic aging was suppressed and excellent iron loss characteristics were obtained (“◯” in Table 4). On the other hand, when the iron loss difference ΔW exceeded 5%, it was determined that magnetic aging occurred and the iron loss characteristics were lowered (“X” in Table 4).

[Test results]
With reference to Table 4, in all of Test Nos. 1 to 32, the chemical composition of the slab was appropriate, and the conditions in each manufacturing process were also appropriate. Also, the plate thickness was 0.20 mm or 0.18 mm. As a result, in any test number, the magnetic flux density B8 was as high as 1.905 T or more, and the Goss direction integration degree B8/Bs was as high as 0.958 or more. Furthermore, the iron loss W _17/50 was as low as 0.821 W/kg or less. Further, the iron loss difference ΔW obtained by the magnetic aging test was all 1% or less, indicating that the magnetic aging was sufficiently suppressed and excellent iron loss characteristics were obtained. Furthermore, the iron loss W 17/50 of these test numbers was significantly lower than the iron loss W _17/50 of the sheet thickness of _0.23 mm shown in Table 2.

In all of test numbers 17 to 32, the chemical composition of the slab was appropriate, and the conditions in each manufacturing process were also appropriate. Also, the plate thickness was 0.18 mm. Therefore, the iron loss W _17/50 of test numbers 17 to 32 is the same as that of test numbers 1 to 16 in Table 4 with a plate thickness of 0.20 mm, with the same Si content and heating rate. Compared with the loss W _17/50 , it showed a lower value and showed excellent core loss characteristics.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (4)

chemical composition is
in % by mass,
C: 0.020 to 0.100%,
Si: 3.35 to 3.75%,
Mn: 0.010-0.300%,
S and / or Se: 0.001 to 0.050% in total,
sol. Al: 0.010-0.065%,
N: 0.002 to 0.015%,
Sn: 0 to 0.500%,
Cr: 0 to 0.500%,
Cu: 0 to 0.500%, and
Balance: Fe and impurities,
A hot rolling process for manufacturing a steel plate by performing hot rolling on a slab consisting of
A cold rolling step of performing cold rolling one or more times on the steel plate after the hot rolling step;
An annealing step before the final cold rolling, in which the steel sheet before the final cold rolling is annealed among the one or more cold rollings;
a temperature rising step of heating the steel sheet after the cold rolling process to a decarburization annealing temperature, heating the steel sheet in a temperature range of 500 to 700 ° C. at an average temperature rising rate of 800 to 2400 ° C./sec; a decarburization annealing step of holding the steel sheet at a decarburization annealing temperature of ~950°C to perform the decarburization annealing;
An annealing separator application step of applying an annealing separator to the surface of the steel sheet after the decarburization annealing step;
A finish annealing step of performing finish annealing on the steel plate coated with the annealing separator;
A method for manufacturing a grain-oriented electrical steel sheet. A method for manufacturing a grain-oriented electrical steel sheet according to claim 1,
The chemical composition of said slab is:
Sn: 0.010 to 0.500%,
Cr: 0.010 to 0.500%, and
Cu: 0.010-0.500%,
A method for producing a grain-oriented electrical steel sheet containing one or more selected from the group consisting of A method for manufacturing a grain-oriented electrical steel sheet according to claim 1 or 2,
A method for producing a grain-oriented electrical steel sheet, wherein in the temperature raising step of the decarburization annealing step, the oxygen potential in the atmosphere is set to 0.001 or less until the temperature of the steel sheet reaches at least 500 to 700°C. A method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 3,
A method for producing a grain-oriented electrical steel sheet, wherein the grain-oriented electrical steel sheet has a thickness of 0.18 to 0.23 mm.

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